1. Field of the Invention
The present invention relates to a variable magnification lens in which imaging magnification can be changed in accordance with a photographing purpose, and to an optical system in which an image recorded on a film can be photographed at magnification most suitable for the film and an optical device, such as an image transforming device, using this optical system.
2. Description of Related Art
Image forming optical systems which are bilateral telecentric and are capable of changing the imaging magnification are proposed, for example, by Japanese Patent Kokai No. 2001-27726 and Japanese Patent No.2731481.
The optical system proposed by Kokai No. 2001-27726 includes, in order from the object side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, a third lens unit with negative refracting power, and a fourth lens unit with positive refracting power. It is constructed as an optical system which is bilateral telecentric and is capable of changing the imaging magnification.
In this optical system, however, when the imaging magnification is changed, an object-to-image distance is varied, and thus there is the need to move the entire optical system in accordance with a change of the magnification.
The optical system proposed by Patent No. 2731481 includes, in order from the object side, a first lens unit with positive refracting power, a second lens unit with negative refracting power, and a third lens unit with positive refracting power. It is constructed as an optical system which is bilateral telecentric and changes the imaging magnification while constantly keeping the object-to-image distance.
In this optical system, however, its F-number fluctuates considerably, depending on the imaging magnification, for example, so that when the imaging magnification is 0.25xc3x97, an image-side F-number is 8.741 and when the imaging magnification is 1.00xc3x97, the image-side F-number is 14.286. Therefore, the problem arises that when the imaging magnification is changed, the brightness of a camera must be adjusted accordingly.
It is, therefore, an object of the present invention to provide an image forming optical system in which even when the imaging magnification is changed, the object-to-image distance remains unchanged and there is little fluctuation in F-number.
In order to accomplish this object, the image forming optical system according to the present invention includes, in order from the object side toward the image side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, a third lens unit with negative refracting power, a fourth lens unit with positive refracting power, and an aperture stop interposed between the third lens unit and the fourth lens unit. The image forming optical system has a variable magnification optical system in which spacings between the first lens unit and the second lens unit, between the second lens unit and the third lens unit, and between the third lens unit and the fourth lens unit are changed to vary the imaging magnification. In this case, the image forming optical system changes the imaging magnification while constantly keeping the object-to-image distance thereof, and in at least one variable magnification state where the imaging magnification is changed, satisfies the following conditions:
|En|/L greater than 0.4
|Ex|/|L/xcex2| greater than 0.4
where En is a distance from a first lens surface on the object side of the variable magnification optical system to the entrance pupil of the image forming optical system, L is the object-to-image distance of the image forming optical system, Ex is a distance from the last lens surface on the image side of the variable magnification optical system to the exit pupil of the image forming optical system, and xcex2 is the magnification of the whole of the image forming optical system.
The image forming optical system of the present invention also satisfies the following conditions:
1.0 less than MAXFNO less than 8.0
|xcex94FNO/xcex94xcex2| less than 5
where MAXFNO is an object-side F-number which is smallest when the imaging magnification of the image forming optical system is changed, xcex94FNO is a difference between the object-side F-number at the minimum magnification of the whole of the image forming optical system and that at the maximum magnification of the whole of the image forming optical system, and xcex94xcex2 is a difference between the minimum magnification of the whole of the image forming optical system and the maximum magnification of the whole of the image forming optical system.
The image forming optical system of the present invention further satisfies the following condition:
0.6 less than |(R3F+R3b)/(R3fxe2x88x92R3b)| less than 5.0
where R3f is the radius of curvature of the most object-side surface of the third lens unit and R3b is the radius of curvature of the most image-side surface of the third lens unit.
The optical device of the present invention uses the image forming optical system of the present invention.
According to the present invention, the image forming optical system in which even when the imaging magnification is changed, the object-to-image distance remains unchanged and there is little fluctuation in F-number, and the optical device using this image forming optical system, can be obtained.
This and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.